This invention relates generally to electric discharge lamps and, more particularly, to flash tubes of the type useful for photographic and photocopying applications.
Flash tubes generally comprise two spaced apart electrodes within an hermetically sealed glass envelope having a rare gas fill, typically xenon, at a subatmospheric pressure. Such lamps are connected across a large capacitor charged to a substantial potential, which is, however, insufficient to ionize the xenon gas fill. Upon application of an additional pulse of sufficient voltage, the xenon is ionized and an electric arc is formed between the two electrodes, discharging the large capacitor through the flash tube, which emits a burst of intense light. In many cases the pulse voltage is applied between an external trigger wire wrapped around the envelope and the electrodes; this is referred to as shunt triggering. However, in other cases an external wire is not feasible since it may result in an undesirable arcing between the trigger wire and a proximate lamp reflector or else the high potential applied to the external trigger wire might be hazardous to operating personnel. In those cases, the lamp may be internally triggered by applying the pulse voltage directly across the lamp electrodes, a technique referred to as injection triggering. Usually the voltage required is about 30 to 50 percent higher than that required to trigger the same lamp with external trigger wire.
In reprographic applications wherein a very uniform light pattern over a relatively large area is desired, shunt-triggered flash tubes having a very elongated envelope have been employed. For example, the envelope may have a somewhat circular configuration with the electrodes disposed at the closed ends of the envelope. Such an arrangement poses a disadvantage in that the long arc discharge path results in a flash duration which is longer than desired for phtographic and photocopying applications. To shorten the flash duration, the use of two semicircular flash tubes has been proposed; however, this increases the amount of dark space which appears at the ends of the tubes where the electrodes are disposed and, thus, detracts from the desired uniformity of illumination.
Photocopy machines have also employed shunt-triggered flash tubes to provide a light source arranged in a somewhat rectangular configuration. More specifically, a pair of U-shaped flash tubes may be employed to form the rectangle, or to provide a shorter flash duration, a system of four L-shaped flash tubes can be connected in parallel. Such an arrangement, however, increases the problem of dark spaces in the light source in the areas where the electrode connections are provided, with a resulting non-uniform illumination in the photocopy application.